The tendencies of iron or steel surfaces to corrode is well known. Zinc is one of the most widely used metallic coatings applied to steel surfaces to protect them from corrosion. In the past, the principal methods of applying such coatings were hot-dipping, also known as galvanizing; and the electroplating of a zinc layer onto the steel. The hot-dip method, while inexpensive and easily applied, resulted in the coating having a thickness of 0.001 inch or more. These coatings, at the temperatures of application, have a tendency to partially alloy at the interface with the steel substrate. The interface alloys are brittle and as a result so-coated materials are not suitable for many forming and finishing operations.
Electroplated zinc produces thinner coatings, about one-tenth the thickness of the hot-dipped coatings, and, it is applied at lower temperatures, causes little or no alloying at the interface between the electroplated zinc layer and the steel substrate. Where rigorous forming and finishing steps are required, such as hot or cold drawing, it is preferred to apply the corrosion-resistant coating by electroplating.
Zinc has been electroplated on the steel surfaces from various plating baths, preferably from acid plating baths, for providing protection of steel surfaces for various uses. The electroplated steel is used for so many varied purposes that the zinc is usually applied to continuous steel strips which, after being plated, are then fabricated into the final articles of manufacture by the conventional cutting, stamping, drawing, forming and finishing operations. However, pure zinc when very thinly applied to steel provides only minimal corrosion protection.
It has been known as in the U.S. Pat. No. 2,419,231 to Shanz, owned by the predecessor of the present assignee, to improve the corrosion resistance of the coating layer by using for the coating an alloy high in zinc and low in nickel. This alloy is co-deposited from the electrolytic plating bath onto the steel substrate. The co-deposition of the high zinc/low nickel alloy is provided by the addition of nickel salts to an acidic zinc-plating bath and then plating at current densities above about 25 amperes per square foot. It was noted that such a plated coating on steel provides superior corrosion resistance to that provided by pure zinc alone.
The nickel/zinc alloy compositions suggested by Shanz range from 10 percent to 24 percent nickel with the remainder zinc. To promote adherence of these nickel-zinc alloys ranging in nickel content from 10 percent to 24 percent with 11 percent to 18 percent nickel being preferred, Shanz recommends that the steel surface first be primed with a thin coating of substantially pure nickel ranging from 0.000025 to 0.00010 inches in thickness. In addition to the improved adherance of the plated alloy, Shanz postulates that some degree of protection against corrosion is provided by the pure nickel "strike" layer since nickel is electronegative to steel and probably at least slows down the electrolytic action between the anodic alloy and the base metal where the latter is exposed. For many years the Shanz co-deposition procedure was followed, usually without the nickel strike layer.
An improvement on the aforementioned Shanz procedure was provided by Roehl in U.S. Pat. No. 3,420,754 also commonly owned. Roehl pointed out that the alloy range used by Shanz for corrosion resistance was an alloy which in addition to being poorly adherant was also insufficiently ductile. Continuous steel strip, alloy-plated in accordance with the teaching of Shanz, when subjected to forming and finishing operations tended to form cracks in the coating because of the brittleness of the alloy. Its relatively high internal stress was the postulated reason. Roehl proposed to solve this shortcoming of the Shanz alloy by restricting the co-deposition to alloys containing less than 10% of nickel. Roehl stated that with less than 10% nickel in the alloy, the plated alloy coatings were more ductile and thus the reduced stress concentration provided a more suitable steel strip for forming and drawing operations.
A subsequent improvement by Roehl et al. in U.S. Pat. No. 3,558,442 also commonly assigned, is based on the stated premise that an improvement in corrosion resistance of the low nickel alloy of the Roehl U.S. Pat. No. 3,420,754 could be obtained if the nickel content of the electro-deposited alloy were slightly increased to a maximum of about 12.5% nickel if deposited from an alloy plating bath maintained at a specific pH range 4.0 to 4.5. This alloy deposited from such baths would still adhere directly to the steel substrate and would still provide corrosion-resistant alloy coating on the steel having sufficient ductility to permit conventional forming and finishing operations. Roehl et al. postulated that while the corrosion-resistance on the stressed specimens was slightly decreased due to the higher nickel content, the "deposit stress" would remain within the acceptable limit previously unavailable for the same alloys deposited from other baths having other compositions and under different pH conditions.
The aforementioned commonly owned Roehl and Roehl et al. patents have been the industrial standards for providing nickel-zinc alloy corrosion-protection to continuous steel strip and other steel substrates.
However, as in all matters pertaining to corrosion-resistance, any expedient which lengthens the corrosion-resistance of the article is a desirable improvement.
It has been noted that considerable variations in the composition of the deposited alloy have been noted. Apparently these are caused by variations of the current density during the plating operation.
Further, at very high plating current densities there is a tendency of the alloy deposit to assume a "burned" texture or quality.
When utilizing the baths of the prior art under the conditions recommended in the Roehl patent, it was also found that when any interruption in the "continuous" plating operation or when the strip was immersed in the plating baths without plating current or if the plated strip, wet with the bath were exposed to air, a dark stain formed, due probably to an immersion-deposit of oxidized nickel salts on the alloy surface. While under normal running conditions, these were not a serious problem, however when the plating line was stopped due to production contingencies an objectionable stain rapidly formed which devalued the resultant product.
The baths utilized in the above-mentioned prior art ranged from seven to nine ounces of nickel (as the metal) per gallon used by Shanz, to from four to five ounces of nickel per gallon in the Roehl and Roehl et al. patents. In addition, the Shanz patent provided a total maximum metal content (nickel plus zinc) of 18 ounces per gallon whereas in the Roehl and Roehl et al. patents the total metal content ranged up to 14 to 15 ounces per gallon. The ratios of nickel:zinc used in the Shanz patent ranged from 0.77:1 to 1.3:1. The Roehl and Roehl et al. patents recommend ratio ranges of 0.40:1 to 0.625:1 and 0.44:1 to 0.7 respectively.